1 One of the largest rodents that ever lived was the giant beaver. Its fossils suggest it was about the size of today's black bear. Giant beavers lived in North America until the last Ice Age. They were believed to be over eight feet long and weigh between two and four hundred pounds.2 Fossils indicate the giant beaver looked similar to the beaver of today, only several times larger. The giant beaver had very large hind feet, which scientists assume were webbed. Its tail appeared to be longer and narrower in size compared to today's beaver.